There are numerous sources of natural or synthetic gases known in the art, and almost all of them contain hydrogen sulfide in various quantities that require at least partial desulfurization prior to further processing or release into the atmosphere. For example, natural gas, refinery gas, synthesis gas (e.g., from gasification of residual oil or coke), or Fischer-Tropsch gas-to-liquids process gases often contain hydrogen sulfide in significant amounts that would interfere with downstream processes. In another example, the sulfur content in the tail gas from Claus reactors typically necessitates treatment before venting the tail gas into the atmosphere.
If hydrogen sulfide is present in relatively large quantities, sulfur removal is commonly accomplished by absorption in an alkaline amine solvent. The so absorbed hydrogen sulfide is desorbed in a regenerator or stripper that operates at a lower pressure and elevated temperature. The acid gas from the regenerator or stripper is then often processed in a Claus plant where the hydrogen sulfide is converted to elemental sulfur by sub-stoichiometric reaction with air or oxygen.
However, nearly all gas streams that contain hydrogen sulfide also contain significant quantities of carbon dioxide, and when such gases are scrubbed with an alkaline solution, carbon dioxide is co-absorbed with the hydrogen sulfide. Co-absorption is particularly problematic where the ratio of carbon dioxide to hydrogen sulfide in the gas stream is relatively high, as such removal will often produce an acid gas with a relatively low concentration of hydrogen sulfide, which tends to cause various problems in the Claus plant. Among other things, dilution of hydrogen sulfide with relatively large quantities of carbon dioxide lowers the net heating value in the acid gas, thereby reducing the residence time in the Claus furnace, which in turn renders sulfur conversion difficult. Moreover, such acid gases typically contain significant quantities of contaminants (e.g., benzene, toluene, xylene and heavy hydrocarbons) that must be destroyed in the Claus furnace to protect the downstream Claus reactor catalyst. However, relatively high carbon dioxide concentrations tend to lower the furnace flame temperature, thereby often making thermal destruction of these contaminants difficult. Moreover, a relatively high carbon dioxide concentration increases the size of Claus plant components as the configuration of a Claus plant is predominantly controlled by the total volumetric flow of acid gas.
In some cases, where the hydrogen sulfide content in the acid gas is below 10%, the Claus process often becomes ineffective and additional and costly processing steps must be employed to enhance the conversion (e.g., using an oxygen or oxygen enriched process). Where the acid gas is entirely unsuitable as a feed to a Claus plant, preprocessing using a selective acid gas removal process is frequently necessary. In a typical preprocessing operation, two gas streams are produced via selective absorption of hydrogen sulfide from the acid gas and subsequent stripping of the rich solvent. Thus, one gas stream predominantly will comprise carbon dioxide and ppmv levels of hydrogen sulfide (suitable for combustion and/or discharge). The other gas stream is enriched in hydrogen sulfide and is processed in a Claus plant using highly reactive catalysts and/or a tail gas treatment unit to meet current emission standards.
Currently known processes for selective removal of hydrogen sulfide from gases with relatively high carbon dioxide concentration include the Stretford, LOCAT, and/or Sulferox processes, which typically employ complex catalyst-based chemistry to directly oxidize hydrogen sulfide to sulfur. However, these units are often complex, difficult to operate, and limited to relatively small capacity. Alternatively, amine based solvents with selectivity towards hydrogen sulfide can be used. For example, selective absorption processes can be based on variations of tertiary amines (e.g., those comprising formulated methyldiethanol-amine (MDEA), sterically hindered tertiary amines, and other amine-organic solvent blends). Such solvents, particularly when combined with special absorber internal designs will minimize co-absorption of carbon dioxide while concentrating hydrogen sulfide three to five fold.
In still further attempts to increase selectivity of absorption, certain tray configurations can be employed to reduce the contact time of the solvent with carbon dioxide (relative to that with hydrogen sulfide) to achieve the desired selectivity (see e.g., U.S. Pat. Nos. 4,278,621, 4,297,329, and 4,678,648). Unfortunately, the use of such processes and/or devices in most cases is not effective and produces only insignificant economic benefit for treating a diluted acid gas stream. Alternatively, as exemplified in U.S. Pat. No. 5,716,587, selectivity is achieved by combining a portion of the contaminants or acid gases from the solvent regenerator to the feed gas to increase the partial pressure of the contaminants at the absorber. While this configuration will marginally increase the concentration of hydrogen sulfide in the resultant acid gas, mixing of two different gases (i.e., the feed gas and the recycled acid gas) having varying hydrogen sulfide concentrations will typically result in a loss of efficiency. Moreover, the use of a single absorber in treating the combined gas requires a high solvent circulation, adding to the cost-ineffectiveness of such configurations.
Alternatively, as described in U.S. Pat. Nos. 4,198,386 and 4,093,701, selectivity is achieved by varying gas flow-rates using a plurality of absorption columns, and splitting the absorber column into a number of absorption zones with controlled flow-rates of lean amine solvent. However, such systems are often costly to install and complicated to operate. In yet further attempts to increase selectivity, hydrogen sulfide absorption may be enhanced via temperature control. Generally, a reduction in absorption temperature slows the carbon dioxide absorption rate. However, the cost and complexity of operating a refrigeration unit renders such an option often uneconomical.
Even where selective acid gas absorption using concurrent conversion of hydrogen sulfide to elemental sulfur in a Claus plant is employed, residual sulfur content in the Claus plant tail gas often poses additional problems. Among other things, the Claus plant tail gas frequently contains substantial quantities of hydrogen sulfide and therefore fails to meet the environmental emission standards for discharging into the atmosphere. There are various configurations known in the art to reduce the sulfur content of a Claus plant tail gas. However, most the known configurations are relatively complex and expensive to operate.
Therefore, and especially where a diluted acid gas feed is encountered, currently known methods and configurations are often neither suitable nor economical. Consequently, there is still a need to provide improved configurations and methods for selective acid gas enrichment.